[Penthode]

I finally got around to rewinding the quadrature injection transformer (T20) on the Zenith. Recall it suffered from heat and crumbled when I attempted to adjust it.

I found an appropriate coil form from an old unused RCA AM/FM radio IF transformer. I had to make two attempts at rewinding because of a misassumption the first time I tried which resulted in initial failure. I could only achieve proper quadrature demodulation with the demodulation axis rotated 90 degrees! I could get a decent picture with relatively good color if I swapped the B-Y for R-Y, G-Y for B-Y and R-Y for G-Y and touch up the quadrature injection. The only difference from the original Zenith transformer and mine was the input coupling winding (pins 2 and 7 on the transformer). I simply bunch wound the winding.

The original winding was a "honeycomb" mesh winding. I reckoned there was too much distributed capacity in my bunch winding. The bunch winding I believe was bringing the primary winding towards resonance: this would add the 90 degrees shift. Fortunately the AM winding from the IF transformer I dissected had two similar honeycomb windings. I used one to determine the total number of turns at 110 per winding and the second I removed the difference to provide the 37 turns I required. I assembled the transformer so that it appeared almost identical from the original.

With my second transformer reinstalled, I was able to get the correct quadrature alignment and demodulation phase with no problem.

The color lock was poor after completing the color oscillator adjustment. Probing with the scope, The burst amplitude low due to the a bad 6JC6 burst amplifier. I found the color oscillator would intermittently lock in phase 180 degrees out! I could find nothing amiss and decided to change the 6KT8 oscillator/reactance control tube for the heck of it. Color lock was now excellent. (I decided to replace all the 6KT8s in the set because I do not trust the critters).

I really like the picture on this set: it is crisper than my RCA's and I really like the Zenith picture. I did not bother with Video IF/ Sound IF or chroma channel alignment because the picture and sound are good. The picture indeed looks better than the photos. The set is using the original Zenith CRT.

The CRT looks to have had a hard life but appears to have a lot of life remaining. Sure glad it wasn't necked!